In certain applications, it is necessary to carry out electrochemical reactions in electrochemical compartments having small dimensions so as to maximize the ratio of the surface area of the reaction fluids in contact with the electrodes to the volume of reaction of fluid delimited by this surface. Such is the case in particular when the reaction involves the formation of high concentrations of unstable intermediate products and/or the use of expensive reagents. Such is also the case when it is desirable to minimize the electrical losses due to the Joule effect through the reaction fluids, or when the size constraints of the reactor are stringent in the context of the application (for example for reactors intended to be introduced into the human or animal body).
Examples of such applications which may be mentioned are the enzymatic electrochemical synthesis of diastereoisomers which require very expensive cofactors (such as NAD (nicotinamide adenine dinucleotide) or NADP (nicotinamide adenine dinucleotide phosphate) in oxidized form (NAD, NAD+, NADP, NADP+) or, above all, in reduced form (NADH, NADPH)) which have to be generated in situ with the aid of mediators (in particular redox mediators such FAD (flavin adenine dinucleotide) or acetyl-CoA (acylation reactions), or PAPS (3′-phosphoadenosine-5′-phosphosulfate), etc.
One of the technological solutions envisaged in this scope consists in using a reactor of the so-called microstructured type, or microreactor, and in particular with electrochemical reaction compartment(s) in the form of a channel or a plurality of parallel channels referred to as microchannel(s), of small cross section making it possible to minimize the transverse gradients of concentrations and electrical potential. The term “microchannel” refers to a channel having transverse dimensions all less than 1 mm.
U.S. Pat. No. 6,607,655 for instance describes an electrochemical reactor having electrochemical compartments with microchannels of rectangular cross section having a height less than 200 μm—preferably between 1 μm and 100 μm—and a width between 5 μm and 1 μm. In one embodiment, the microchannels are formed by grooves made on the surface of the electrodes.
Nevertheless, the teaching given by this document in this regard remains purely theoretical in so far as, on the one hand, the method for manufacturing such microstructured grooves is not given and, on the other hand, flows in microchannels with dimensions corresponding to the lower thresholds of the indicated dimensions would be accompanied, for liquids which perfectly wet the corresponding metal surface, by large pressure drops (more than 5.105 Pa) which are incompatible with the use of compositions such as biological compositions liable to experience a loss of activity under excessive pressures.
Until now, only cross sections of rectangular or trapezoidal shape have been envisaged for the production of microchannels in microstructured reactors. This is because the etching and/or deposition methods which may be envisaged for producing such microchannels (for example photolithographic and screen printing methods, physical or chemical depositions, etc.) necessarily lead to such polygonal shapes with sharp corners.
However, the inventors have determined that the presence of corners, or more generally pronounced curvature variations, in the cross section of the microchannels etched in an electrode considerably impair the performances of such an electrochemical reactor, at least for the following two reasons:                it locally (in the microchannel) induces corresponding distortions of the electrical field generated by the electrode, and therefore a significant heterogeneity of the electrochemical reaction mechanism, which proves fatal in practice in view of the very small transverse dimensions and the extremely sensitive nature of the reactor (because of the great instability of the intermediate products);        in view of the wetting phenomena which assume great importance with sub-millimeter dimensions, it considerably impairs the flow of the fluid in the microchannel by tending to cause shedding of the boundary layer and inducing a very significant resistance to the flow, making it impossible in practice to use microchannels with a cross section less than 20000 μm2 with liquids whose viscosity and surface tension are similar to those of water.        
With the dimensions indicated in U.S. Pat. No. 6,607,655 and for a Peclet number less than 1000, the thickness of the diffusion layer is thus smaller than the transverse dimensions of the channel, which introduces a transverse concentration gradient that contributes to a nonuniform distribution of the electrical field. The consequence of these gradients is a low selectivity, in particular for the applications to electro-enzymatic reactions as mentioned above. For example, the concentration of the reduced form of a mediator such as flavin is not sufficient anywhere in the channel (on account of this mediator's stability) for the synthesis reaction to be spontaneous. Furthermore, a nonuniform distribution of the electrical field can induce the transformation of several chemical functions of an electro-active species, and not just specifically the target part of the molecule. Side-effects may therefore occur.
It should be noted in this regard that these problems are specific to electrochemical reactions and, in particular, are not encountered in the case of purely chemical reactions.